The present invention relates to a method for recrystallizing conductive material films including semiconductors, particularly to a process for an SOI (Silicon on Insulator) technology.
SOI technology is proposed as means for providing high-speed and high-voltage semiconductor integrated circuits (ICs) and is expected as one of the most promissing means for materializing three-dimensional semiconductor integrated circuits. In SOI technology, a polysilicon film is usually formed on an insulating layer such as a thermally-oxidized layer of a silicon substrate and is recrystallized to be single crystalline or grain-boundary free by using irradiation of a radiant energy beam such as a laser. To facilitate the transformation of the polysilicon film into a single crystal film, the nucleation in the polysilicon film during the recrystallization must be controlled to initiate at a single point in the molten region of the polysilicon film. Such condition can be achieved by producing a temperature distribution in the region, which has a profile that is lowest at the center of the region and becoming higher and higher toward the periphery of the region.
There are many reports disclosing methods to establish such temperature profile, including "Recrystallization of Si on amorphous substrate by doughnut-shaped cw Ar laser beam" by S. Kawamura et al., Applied Physics Letter 40 (5), p. 394, 1 March 1982. The doughnut-shaped laser beam, for example, produces a temperature profile the lowest at the position corresponding to the center of the beam. In the United States patent application, Ser. No. 784,607, the present inventor discloses a selective recrystallization of a polysilicon film, using an anti-reflective film having apertures formed therein at the positions corresponding to the regions to be single-crystalline in the polysilicon film. With the apertures, the anti-reflective film produces a desired temperature profile which is the lowest at the center of each region corresponding to the apertures. According to this method, any desired portion of a polysilicon film can be transformed into a single crystalline region of a desired size. These disclosures are categorized as a direct-heating-type SOI technology.
In the United States Pat. No. 4,543,133, published Sep. 24, 1985, and a report entitled as "Single crystalline Si islands on an amorphous insulating layer recrystallized by an indirect laser heating technique for three-dimensional integrated circuits" on pp. 994-996 of Applied Physics Letter 44 (10), 15 May 1984, the present inventor also discloses an SOI technology utilizing an indirect heating of a polysilicon film. In the disclosure, islands of a polysilicon film and the surrounding substrate region are coated with an energy-absorbing cap layer. The energy-absorbing cap layer is irradiated with a radiant energy beam such as an argon ion laser and generates heat. Thus, the polysilicon film islands are melted by the heat transferred from the energy-absorbing cap layer by the thermal conduction.
In such indirect heating SOI technology, the fluctuations in the output power and the intensity distribution of the laser beam are buffered by the energy-absorbing cap layer, and hence, stable and uniform heating can be attained. As a result, the polysilicon islands can be recrystallized to be grain-boundary-free with an improved reproducibility. Further, the indirect heating SOI technology allows the radiant energy beam to be free from the optical absorption characteristics of the to-be-recrystallized film such as polysilicon film and only requires the matching between the wave length of the radiant energy beam such as laser beam and the absorption spectrum of the energy-absorbing cap layer.
However, in the above-mentioned indirect heating SOI technology, the polysilicon film formed on an insulating layer is patterned to provide the islands thereof prior to the recrystallization. The size of each island must be large enough to include therein the source and drain regions together with the active region, i.e. channel region, of a transistor.
Generally, it is difficult to establish the above-mentioned desired temperature profile in the island as the size of the island becomes large. This means that, in the prior art indirect-heating SOI technology, there exist islands each being recrystallized to have a grain boundary with a relatively high probability. If there is an island having a grain boundary extending to the active region thereof in a number of islands on a semiconductor chip, the chip can not any longer be used for an integrated circuit, and thus, the yield of integrated circuit chips based on the indirect-heating SOI technology is decreased.
As described above, it is advantageous that a region to be recrystallized is as small as possible when a desired temperature profile is established therein. Accordingly, it is desired to selectively recrystallize only a small region such as the channel region of a transistor, and the source and drain regions are left in polycrystalline state. However, the prior art indirect-heating SOI technology is unable to selectively recrystallize only such a small region as the channel region in a polycrystalline island.